Creep-resistant, high-strength silicon carbide fibers

ABSTRACT

A high strength, high creep resistant, boron-doped, silicon carbon fiber having no boron nitride coating, originally formed by sintering, is produced by exposing the fiber to a nitrogen atmosphere at a temperature equal to or preferably elevated above the sintering temperature and also exposing the fiber to a carbon monoxide-containing atmosphere at a temperature sufficient to remove boron and boron nitride. The nitrogen atmosphere step may be performed before or after the carbon monoxide-containing atmosphere step. The resulting, uncoated SiC fibers have tensile strengths greater than approximately 2.0 GPa and Morscher-DiCarlo BSR test creep resistance M values greater than approximately 0.75 at 1400 degrees C. for one hour in argon. The method is applicable to non-sintered fibers as well, in which case the nitrogen exposure is carried out at between approximately 1750 to 2250 degrees C. and the carbon monoxide exposure is carried out at between approximately 1600 to 2200 degrees C.

This invention was made with government support under grant/contractnumber F49620-94-1-0429 awarded by the Air Force Office of ScientificResearch. The government has certain rights in the invention.

This application claims the benefit of U.S. Provisional Application No.60/055,422, filed Aug. 4, 1997.

BACKGROUND OF THE INVENTION

Silicon carbide (SiC) is a material with excellent mechanical propertiesat high temperatures. SiC fibers are of great interest for thefabrication of composite materials, especially composite materials foruse in high temperature structural applications. This invention concernsa method for producing silicon carbide (SiC)-based fibers with improvedcreep resistance and high tensile strength, and the fibers so produced,by exposing the sintered fibers to nitrogen gas at temperatures abovethe sintering temperature and exposing the fibers to carbon monoxide gasat lower temperature, with the exposure steps occurring in either order.

There are many methods of forming SiC ceramics. SiC ceramics are oftenprepared by forming and consolidating fine SiC particles into a desiredshape and subsequently heat treating (i.e., sintering) the "green" shapein order to eliminate the interparticle pores (i.e., void space) and toobtain a high-strength body with high relative density (i.e., withlittle or no residual porosity).

SiC ceramics are also prepared by other methods, especially by chemicalvapor deposition (CVD) and by heat treatment of organosilicon polymers.For example, organosilicon polymers have been used to prepare fine SiCparticles, fibers, bulk samples, coatings, etc. Continuous SiC fiberswith fine diameters (e.g., ˜5-25 μm) are generally prepared fromorganosilicon polymers. Some SiC fibers (and other types of samples)prepared using organosilicon polymers develop fine SiC crystallites andfine pores at some stage during processing and, hence, a sintering stepis required to ultimately produce a dense (low porosity), high-strengthSiC sample.

The typical sintering temperatures for preparing dense SiC are in therange of approximately 1700-2300° C. The required temperature is highlydependent upon the size of the SiC particles (or crystallites) whichcomprise the porous body that is being sintered. For example, SiC bodiesfabricated from the more conventional powder processing routes generallyrequire higher sintering temperatures (˜1900-2300° C.) and this resultsin sintered bodies comprised of coarser grain sizes (>1 μm). Incontrast, organosilicon polymer-derived SiC bodies (e.g., fibers) can besintered at lower temperatures (e.g., ˜1700-1900° C.) and, consequently,the resulting grain sizes are usually smaller (<1 μm).

For high temperature applications, it would be desirable to have SiCfibers with high strength and high resistance to creep. As discussed byDiCarlo (in Composites Science and Technology, 51 213-222, 1994), it isdifficult to achieve both of these properties simultaneously in SiCfibers. The strength of SiC fibers is controlled by "flaws" (e.g.,voids, particulate impurities, kinks, grains, etc.). When other flawsare avoided through careful processing, the strength is controlled bythe size of the SiC grains comprising the fibers. Hence, it is generallyobserved that carefully processed organosilicon polymer-derived SiCfibers have much better strength compared to powder-derived SiC fibersbecause the grain sizes are much smaller. The tensile strengths fororganosilicon polymer-derived SiC fibers are typically in the range of˜2.0-3.5 GPa. (Fiber tensile strengths ≧2 GPa are desirable for thedevelopment of high-strength, fiber-reinforced composites.) In contrast,the tensile strengths for powder-derived SiC fibers are usually muchlower, e.g., ˜0.5-1.5 GPa.

The creep resistance of SiC fibers is usually highly dependent upongrain size also. However, the effect of grain size on creep behavior isthe opposite of its effect on strength. The creep resistance increasesas the grain sizes increases. Hence, powder-derived SiC fibers usuallyshow better creep resistance than polymer-derived SiC fibers. Thus, ithas not been possible in the past to make SiC fibers with both hightensile strength and high creep resistance. The difficulty insimultaneously achieving high tensile strength and high creep resistancein SiC fibers was demonstrated by Takeda et al. (in Ceram. Eng. Sci.Proc., 17[4] 35-42, 1996) using the organosilicon polymer-derived fibersknown as HI-NICALON TYPE S™ which are produced by Nippon Carbon Co.As-prepared HI-NICALON TYPE S™ fibers have an overall composition closeto that of stoichiometric SiC, low-oxygen-content (≦0.2 wt %), hightensile strength (˜3 GPa), and fine diameter (˜12 μm). Takeda et al.heat treated these fibers at temperatures up to 1900° C. for 10 hours inan argon atmosphere. The fiber tensile strengths decreased withincreasing heat treatment temperature, reaching values below 1 GPa forthe longest heat treatment carried out at the highest temperature. X-raydiffraction line broadening measurements showed that the SiC crystallite(grain) sizes increased as the re-heat treatment temperature (and/ortime) increased. Hence, the decrease in tensile strength with heattreatment was attributed to the increase in grain sizes. The creepbehavior of the SiC fibers was assessed using a bend stress relaxation(BSR) test. In contrast to the strength behavior, the creep resistanceof the fibers increased as the grain size increased. Fibers given thelongest heat treatment carried out at the highest temperature showed thebest creep resistance, as assessed by the BSR test. Heat treatments ofthe organosilicon polymer-derived HI-NICALON TYPE S™ fibers at thehighest temperature and longest time resulted in fibers that weresimilar to powder-derived fibers in terms of good creep resistance andlow strength values. This combination was observed because the heattreatments resulted in fibers with relatively large grain sizes.

It is well known that SiC ceramics with high relative density (i.e., lowresidual porosity) and fine grain sizes are desirable in order to attainhigh strength. However, it is very difficult to prepare pure SiC withhigh relative density and fine grain sizes by sintering methods,especially by pressureless sintering methods. In samples comprised offine particles (or fine crystallites), pure SiC generally undergoescoarsening (growth) of particles (crystallites) and pores during hightemperature heat treatment because of the dominance of surface diffusionand/or vapor phase diffusion processes. Thus, very little densification(i.e., pore removal) occurs in pure SiC during high temperature heattreatment. As a result of this problem, additives (i.e., "sinteringaids") are used to enhance the densification (and prevent coarsening)during sintering and, thereby, allow the fabrication of SiC with highrelative density and fine grain sizes. Several additives have been foundeffective as sintering aids for SiC, but boron-containing compounds arethe most commonly used additives. Varying amounts of boron compoundshave been reported as effective for sintering (e.g., 0.2-5 wt %), butboron concentrations on the order of ˜0.5-1 wt % are most common. Whensintering aids are used, the typical sintering temperatures forpreparing dense SiC are in the range of approximately 1700-2300° C. Asnoted earlier, the required temperature is highly dependent upon thesize of the SiC particles (or crystallites) which comprise the porousbody that is being sintered.

In addition to controlling the sintering temperature and using theproper amount and type of sintering additive, it is also important tocontrol the gas atmosphere during sintering of SiC in order to achievehigh relative density and fine grain size. There have been numerousstudies in which SiC was sintered in various atmospheres. It is wellknown that oxidizing atmospheres are undesirable, while atmosphereswhich are usually referred to as "inert" or "chemically inert" areconsidered desirable. Argon is the most common atmosphere used insintering of SiC. Helium, nitrogen, and vacuum have also been widelyreported as useful atmospheres for sintering of SiC.

As described in more detail below, there have been many prior studies inwhich SiC was fabricated using nitrogen as the atmosphere during thesintering (densification) process. In the prior art, however, heattreatments in nitrogen-containing atmospheres have only been carried outat the sintering temperatures or at temperatures below the sinteringtemperatures. Furthermore, the prior art specifically teaches againstheat treating SiC at temperatures above that required for the purposesof sintering. According to the prior art, this will result in a loss ofstrength of the SiC body. In contrast, the current invention teaches thesurprisingly result that high strengths can be maintained in sampleswhich are heat treated in a nitrogen-containing atmosphere attemperatures higher than the sintering temperature.

Prochazka in U.S. Pat. Nos. 3,853,566, 3,960,577, 3,954,483, 3,968,194,and 4,004,934 reported that small amounts of boron (especially whencombined with small amounts of carbon) could be used to enhancedensification and produce SiC bulk samples with high densities and finegrain sizes. Boron was incorporated by mixing SiC powders withboron-containing powders such as elemental boron, boron carbide, andboron nitride. It was also reported that gaseous boron trichloride couldbe used to vapor-phase dope boron directly into SiC powders during theirsynthesis by CVD methods. The minimum amount of boron needed for gooddensification was approximately 0.3 wt %, although higher amounts weregenerally used. Prochazka and many others researchers concluded that itis desirable to use the lowest possible temperature and shortestpossible time that are needed to accomplish densification (i.e.,elimination of porosity) during sintering of SiC. This is not onlybecause of the cost savings, but also because prolonged heat treatmentresults in grain growth in the sintered body. Grain growth is consideredundesirable in many cases because it usually results in a reduction inthe strength of the sintered body. For example, in U.S. Pat. No.4,004,934, Prochazka states: "An extended dwell [time] at hightemperature is harmful because it brings about coarsening of themicrostructure and consequently degradation of the mechanicalproperties. Thus, the shortest hold [at the sintering temperature] ispreferable." Because Prochazka used powder processing methods, sinteringand hot pressing temperatures were generally in the range of ˜1900-2100°C. (with the lower temperatures in this range being used for sampleswhich were hot pressed). From U.S. Pat. Nos. 3,853,566, 3,960,577,3,954,483, 3,968,194, and 4,004,934, it is reported that boron-doped SiCbodies can be sintered to high relative density using several differentgas atmospheres, including argon, helium, and nitrogen. In the firstclaim of U.S. Pat. No. 4,004,934, it is stated that a dense boron-dopedSiC ceramic can be made by "sintering of the green body in an inertatmosphere chemically-inert with respect to silicon carbide atatmospheric pressure or below atmospheric pressure at a temperature ofabout 1900-2100° C. . . . ." A nitrogen atmosphere was used in most ofthe examples in U.S. Pat. No. 4,004,934, so this patent teaches that SiCis chemically-inert in nitrogen.

Many other studies have been reported in which SiC is sintered in anitrogen-containing atmosphere. In each case, the prior art teaches thatthe nitrogen atmosphere should be used at the lowest temperaturerequired for densification. For example, Urasato et al. in U.S. Pat. No.5,011,639 sintered green bodies prepared with SiC powders which had beenmixed with ˜0.2-5.0 wt % of boron powder or a boron compound powder(e.g., boron carbide, boron oxide, titanium diboride). In this case,nitrogen was not considered an inert gas. The patent states that greenbodies were ". . . sintered in an atmosphere of an inert gas containingnitrogen in a minor concentration. The inert gas here implied is a raregas such as helium, argon, and the like. The concentration of nitrogenin the atmosphere of mainly the inert gas should be in the range from0.005 to 5% by volume or, preferably, from 0.01 to 2% by volume." It isstated that "The furnace for sintering should be filled withnitrogen-containing inert gas throughout the sintering procedure." Inaddition, it is stated that "The temperature of sintering should be inthe range of 1800 to 2200° C. or, preferably, from 1950 to 2150° C. Whenthe temperature for sintering is too low, the green body cannot be fullysintered so that the sintered body would have somewhat decreaseddensity. When the temperature of sintering is too high, on the otherhand, undue grain growth and thermal decomposition of silicon carbidemay take place so that the sintered body has a decreased electricresistivity and mechanical strength."

Boron-doped SiC bodies with high relative density have also been formedusing mixtures of SiC particles and borosiloxane polymers. This wasdemonstrated by Burns et al. in U.S. Pat. No. 5,112,779 using sinteringtemperatures in the range of 2000-2200° C. to achieve densification.Nitrogen was considered an inert atmosphere suitable for sintering. Itwas stated that "Inert atmospheres are used for sintering. . . . For thepurposes of this invention, an inert atmosphere is meant to include aninert gas, vacuum, or both. If an inert gas is used it may be, forexample, argon, helium, or nitrogen."

Powder processing methods can be used to form SiC fibers, especiallyfibers with larger diameter (i.e., >25 μm). Frechette et al. in U.S.Pat. No. 4,908,340 and in Ceram. Eng. Sci. Proc. 12[7-8] 992-1006, 1991(by F. Frechette, B. Dover, V. Venkateswaran, and J. Kim) prepared SiCfibers by either melt spinning or dry spinning of mixtures of SiCparticles and organic polymers. Boron was incorporated in the fibers byadding 0.2-1.0 wt % boron carbide powders to the spinning mixtures. Somefibers were heat treated in crucibles which had been coated withslurries containing boron carbide in order to form boron-containinggases during the sintering operation. SiC fibers were sintered attemperatures in the range of 1900-2150° C. in an argon atmosphere or at2300° C. in a nitrogen atmosphere in order to obtain substantially dense(i.e., low porosity) fibers. (It was reported in U.S. Pat. Nos.4,004,934 and 4,908,340 and elsewhere that higher sintering temperatureswere required for densification (i.e., pore removal) when nitrogen wasused as the sintering atmosphere.)

The fibers produced by the method of Frechette at al. are usuallyreferred to as Carborundum SiC fibers in various published scientificpapers. As expected from the earlier discussion, these powder-derivedfibers had relatively low tensile strength (˜1 GPa) because the grainsize were relatively coarse (i.e., average grain sizes were greater than1 μm and individual grains up to ˜10 μm were observed). However, thefibers showed excellent strength retention after heat treatment atelevated temperature. In addition, creep resistance was excellent, asindicated from both BSR tests and from in-situ creep tests carried outat high temperatures.

Bolt et al. in U.S. Pat. No. 4,942,011 prepared SiC fibers by spinningmixtures of SiC particles and organosilicon polymer. One of thesintering aids used was boron carbide. The concentration of thesintering aids was 0.2-5% based on the SiC weight. Fibers were heattreated (sintered) at temperatures in the range of 1900-2000° C. in anargon atmosphere, although it was indicated that inert or reducingatmospheres could be used.

Birchall and Clegg in U.S. Pat. No. 5,063,107 prepared SiC fibers in asimilar manner to Frechette et al. and Bolt et al. They used mixtures ofSiC particles and organic polymers. They also utilized known sinteringaids, such as boron. Sintering was carried out at 2040° C. for 20 min inan argon atmosphere. It was stated in the patent that "The later stageof the process of the invention will generally be effected in an inertatmosphere by which we mean in an atmosphere which is unreactive withthe silicon carbide at the temperature of heating." It is also statedthat "It may also be necessary to avoid the use of nitrogen as nitrogenmay react with the silicon carbide to produce silicon nitride."

Organosilicon polymers are generally used to produce SiC fibers withfine diameter (i.e., <25 μm). Yajima et al. in U.S. Pat. Nos. 4,052,430,4,100,233, 4,220,600, and 4,283,376 developed processes for preparingpolycarbosilane polymers and SiC-based fibers from polycarbosilanepolymers. The processes developed by Yajima et al. are the basis forNICALON™ fibers which are manufactured by Nippon Carbon Co., Ltd. Asimilar commercial process involves using a titanium metal-modifiedpolycarbosilane (i.e., a polytitanocarbosilane) polymer and is the basisfor TYRANNO™ fibers which are manufactured by Ube Industries Ltd. Yajimaet al., in some of the earlier cited patents and in U.S. Pat. No.4,152,509, also reported that SiC fibers could be prepared frompolycarbosilane polymers which were synthesized from a polysilane and aphenyl-containing polyborosiloxane. The organosilicon polymer fibersproduced by the various methods of Yajima et al. are pyrolyticallydecomposed to SiC-based fibers by heat treatment at temperatures usually≦1300° C. Pyrolysis was usually carried out in nitrogen or argonatmospheres or under vacuum. The fibers produced by these methods do nothave good thermomechanical stability. The fibers degrade extensively andbecome extremely weak when heat treated at temperatures aboveapproximately 1200-1400° C. This is because of carbothermal reductionreactions that occur as a result of large amounts of oxygen and excesscarbon in the SiC fibers.

Wallace et al. in U.S. Pat. No. 5,139,871 reported that thethermomechanical stability of organosilicon polymer-derived SiC fibers,such as TYRANNO™ fibers, can be improved by certain heat treatments in anitrogen atmosphere. It was specified that heat treatments could becarried out between 800-1800° C., with a preferred range of 1100-1600°C. It was shown that heat treatments in a nitrogen atmosphere alone didnot result in any improvement in the thermomechanical properties of thefibers. However, a small improvement in the thermomechanical propertiesof the fibers was observed when the fibers were placed in intimatecontact with carbon particles during the heat treatment in nitrogen.Nevertheless, the fibers still had low strengths (i.e., <1.5 GPa) afterheat treatment at only 1600° C.

Takamizawa et al. in U.S. Pat. No. 4,604,367 prepared boron-dopedSiC-based fibers using organoborosilicon polymers which were prepared byreacting an organopolysilane with an organoborazine compound. Theorganoborosilicon polymers were melt spun into green fibers which wereinfusibilized by oxidative or irradiative cross-linking methods andsubsequently pyrolyzed to form SiC-based fiber containing boron andnitrogen. Takamizawa et al. indicated that heat treatments could becarried out in vacuum or inert gas atmospheres at temperatures up to1800° C., although the preferred heat treatment temperatures were <1600°C. Nitrogen was considered a suitable gas for the pyrolysis process. Theresulting fibers showed better strength retention upon high temperatureheat treatments compared to NICALON™ fibers, but the fibers stillstarted to show decreased strengths after heat treatments attemperatures above approximately 1100-1200° C. and the strengths weretoo low for the fibers to be useful at temperatures above approximately1500° C.-1600° C.

Researchers at Dow Corning have reported in U.S. Pat. Nos. 5,071,600,5,162,269, 5,167,881, 5,268,336, 5,279,780 and 5,366,943 the fabricationof boron-doped, low-oxygen-content SiC fibers with carbon-rich ornear-stoichiometric composition which were prepared from severaldifferent organosilicon polymers (i.e., polycarbosilane,methylpolydisilylazane, polyorganosiloxane). In U.S. Pat. Nos. 5,071,600and 5,162,269, for example, fibers were formed by melt spinning oflow-molecular-weight polycarbosilane. Fibers were oxidatively cured toprevent melting of the fibers during subsequent heat treatment. Theprimary modification of the process of Yajima et al. was to incorporatesufficient amounts of boron (>0.2 wt %) in the fibers so that highrelative density and fine grain sizes could be obtained after sintering.The patents describe mostly gas-phase doping methods, in which it wasindicated that boron-containing compounds (e.g., diborane, borontrifluoride, boron tribromide, boron trichloride, tetraborane,pentaborane, borazine, trichloroborazine) could be infiltrated into thefibers at temperatures in which the compounds were in the form of gases.The boron-containing gas could be introduced at various stages in thefiber fabrication process, but it must be present prior to the onset ofsintering in order to prevent grain coarsening and to allowdensification of the porous SiC fibers to occur. In addition to gasphase doping, the Dow Corning patents also describe instances in whichthe organosilicon polymer can be synthesized such that it contains someboron.

The Dow Corning patents indicate that oxygen, nitrogen, and excesscarbon are eliminated from fibers (as gaseous volatiles) during a hightemperature pyrolysis stage which occurs in the range of ˜1200-1600° C.The fibers develop fine pores during this stage. The porosity is removedby sintering at a temperature typically in the range of 1800-1850° C.(As noted above, boron must be present to prevent grain coarsening andallow pore removal to occur.) The Dow Corning patents teach against theuse of nitrogen atmospheres for the high temperature processing steps.In U.S. Pat. No. 5,167,881, it is stated that "At pyrolysis temperaturesabove about 1400° C., nitrogen-containing atmospheres are not preferredas nitrogen is not inert to the fibers under those temperatureconditions. At high temperatures, truly inert gaseous atmospheres arepreferred such as argon and/or helium." A similar statement is made inU.S. Pat. No. 5,268,336. (It should be noted that the patents use theterm "pyrolysis" to encompass not only the process in which volatilesare removed at lower temperatures, but also the higher temperaturesintering process.) The fibers produced in the Dow Corning patents donot contain any nitrogen. The first claim in U.S. Pat. No. 5,162,269states that the fibers are "nitrogen-free." In U.S. Pat. No. 5,268,336,it is stated "The duration of the pyrolysis treatment should besufficient to eliminate substantially all oxygen and/or nitrogen fromthe fibers."

Fiber characterization data, such as reported by Dicarlo (in CompositesScience and Technology 51 213-222, 1994), Takeda et al. (Ceram. Eng.Sci. Proc., 17[4] 35-42, 1996), and Lipowitz et al. (Ceram. Eng. Sci.Proc., 16[4] 55-62, 1995), indicated that SiC fibers produced accordingto the methods described in the Dow Corning patents havethermomechanical stability which is much better than NICALON™ andTYRANNO™ fibers and similar to HI-NICALON TYPE S™ fibers. The fibersretained 96% of their original tensile strength after heat treatment at1550° C. for 10 hours in an argon atmosphere. The fibers retained 74% oftheir original tensile strength after heat treatment at 1800° C. for 12hours in an argon atmosphere. The fibers produced according to the DowCorning patents are known as SYLRAMIC™ fibers.

Sacks et al. in U.S. Pat. No. 5,792,416 reported the fabrication ofboron-doped, low-oxygen-content SiC fibers with near-stoichiometriccomposition, high relative density, and high tensile strength which wereprepared using high-molecular-weight polycarbosilane polymers. Fiberswere sintered in an argon atmosphere. In most examples, the fibers weresintered in the range of 1750-1800° C. for 1 hour. If fibers weresintered at higher temperatures, shorter sintering times were used toproduced fibers with high relative density and high strength. Forexample, sintering times were 12 and 2 minutes when sinteringtemperatures were 1845 and 1890° C., respectively. Sintered SiC fibersproduced by this method had an average tensile strength of ˜2.85 GPa.The fibers typically retained more than 90% of the original tensilestrength after being re-heated in an argon atmosphere at 1800° C. for 4hours. However, the tensile strength decreased to ˜75% of the originalstrength after heat treatment in argon at 1950° C. for 1 hour. Hence,heat treatments in an argon atmosphere which were carried out at highertemperatures or longer times than needed for sintering resulted indecreased tensile strengths of the fibers.

Sacks, in U.S. Provisional Appl. Ser. No. 60/055,424 entitled "SiliconCarbide Fibers with Boron Nitride Coatings," reported that boron-dopedSiC fibers with high relative density (i.e., little or no porosity) canbe heat treated in a nitrogen-containing atmosphere, at a temperaturewhich is higher than the temperature used to originally sinter thefibers, to develop a boron nitride (BN) layer at the SiC fiber surface.In contrast to all previous results with SiC, the heat treatment attemperatures above the sintering temperature did not result in adecrease in the strength. The BN-coated SiC fibers prepared in thismanner still had high tensile strengths, i.e., the strengths wereapproximately the same as the original uncoated SiC fibers.

SUMMARY OF THE INVENTION

A high strength, high creep resistant, boron-doped, silicon carbonfiber, having no boron nitride coating, originally formed by sintering,is produced by exposing the fiber to a nitrogen atmosphere at atemperature at least equal to but preferably elevated above thesintering temperature (approximately between 1750 and 2250 degrees C.for sintered or non-sintered fibers) and also exposing the fiber to acarbon monoxide-containing atmosphere at a temperature approximatelybetween 1600 and 2200 degrees C., more preferably between approximately1700 and 2000 degrees C., and most preferably between 1700 and 1900degrees C. The nitrogen atmosphere step may be performed before or afterthe carbon monoxide-containing atmosphere step. The resulting, uncoatedSiC fibers have tensile strengths greater than approximately 2.0 GPa andMorscher-DiCarlo bend strength relaxation (BSR) creep resistance Mvalues greater than approximately 0.75.

In one embodiment, the SiC fiber is first exposed to a nitrogenatmosphere at a temperature approximately 50 degrees C. above thesintering temperature for a period of approximately 10 minutes. Thetemperature is then lowered approximately 190 degrees C. and thenitrogen atmosphere is replaced with an atmosphere of approximately 15%carbon monoxide and 85% argon, this temperature is maintained for about30 minutes and the fiber then allowed to cool. In a second embodiment,the SiC fiber is first exposed to an atmosphere of approximately 15%carbon monoxide and 85% argon at the sintering temperature for about 50minutes. The atmosphere is then replaced with a nitrogen atmosphere andthe temperature elevated approximately 100 degrees C. above thesintering temperature, maintained for about 30 minutes and the fiberthen allowed to cool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an AES spectrum for the control (uncoated) SiC fiber sample ofExample 1 after heat treatment at 1845° C. for 12 minutes in argon.

FIG. 2 is an AES spectrum for the same control (uncoated) SiC fiberafter it has been subjected to sputtering with argon in order to removethe thin layer of carbon from the SiC fiber surface.

FIG. 3 shows the AES spectrum for a BN-coated SiC fiber of Example 2prepared by heat treatment at 1890° C. for 10 minutes in a nitrogenatmosphere.

FIG. 4 shows the AES spectrum for the BN-coated fiber of FIG. 3 after ithas been subjected to sputtering with argon in order to remove a thinlayer of the SiC fiber surface.

FIG. 5 shows the AES spectrum for the unsputtered surface for aBN-coated SiC fiber of Example 2 prepared by heat treatment at 1940° C.for 1 hour in a nitrogen atmosphere.

FIG. 6 shows the AES spectrum for the unsputtered surface for the SiCfiber of Example 4.

FIG. 7 shows the AES spectrum for the unsputtered surface for the SiCfiber of Example 5.

DETAILED DESCRIPTION OF THE INVENTION

In the current invention, it was discovered that a BN coating is not arequirement for the SiC fibers to have high tensile strength andimproved creep resistance. In this invention, it was determined thatheat treatment carried out using both nitrogen and carbon monoxide canbe used to produce SiC fibers with high tensile strength and high creepresistance, but with no BN coating at the fiber surface.

The initial processing steps for preparing the SiC fibers in thisinvention were the same as in U.S. Pat. No. 5,792,416 by Sacks et al.This includes the following steps: (i) Polycarbosilane (PCS) polymer issynthesized by pressure pyrolysis of polydimethylsilane (PDMS). Theaverage molecular weight for the PCS is typically in the range ofapproximately 7,000-16,000. (ii) A concentrated fiber-spinning solution("spin dope") is prepared using PCS, one or more solvents (e.g.,toluene), one of more "spinning aids" (e.g., polysiloxane (PSO),polysilazane (PSZ)), and a boron-containing sintering aid (e.g., solidboron hydride). The typical range of polymer concentrations are fromapproximately 60 to 75% and the typical spin dope viscosities are in therange of approximately 10 to 100 Pa·s. (iii) Fibers are formed byextruding the spin dope through a spinneret and collecting the extrudedfilaments on a winding wheel. (iv) The "green" (as-collected) fibers areheat treated using oxidizing and non-oxidizing atmospheres to accomplishremoval of solvent and incorporation of oxygen (less than about 300°C.), pyrolytic decomposition of the organosilicon polymer (less thanabout 1200° C.), carbothermic reduction reactions which remove excesscarbon and oxygen (less than about 1650° C.), and sintering to densifythe fiber (less than about 2000° C.). In the last of these initialprocessing steps, the fibers are heat treated in an inert gasatmosphere, such as in argon gas, in order to produce a fiber with highrelative density (i.e., low residual porosity). In the presentinvention, additional processing steps are used. In particular, thesintered fibers are heat treated in a nitrogen gas-containing atmosphereand a carbon monoxide (CO) gas-containing atmosphere.

The heat treatment in a nitrogen gas-containing atmosphere is carriedout at higher temperatures (preferably between about 1750 to 2250degrees C.) and/or longer times than required for densification. Thisheat treatment results in larger grain size which is beneficial forcreep resistance. It is also possible that some nitrogen may beincorporated in the SiC lattice which may be beneficial for the creepresistance. It should be emphasized that the heat treatment in thenitrogen-containing atmosphere is not carried out during the process ofsintering the SiC fibers. As described in detail earlier, it iswell-known that nitrogen has been used as the atmosphere for sinteringof SiC fibers and other SiC bodies. The present invention differs inthat the heat treatment in the nitrogen-containing atmosphere occursafter the sintering (densification) process and at a higher temperatureand/or longer time than used for sintering (densification). Some of theprior art indicates that the use of nitrogen atmosphere during thesintering process may be detrimental to the preparation of SiC fiberswith high tensile strength. A similar observation is reported in thisinvention. The prior art also indicates that heat treatments above thetemperature needed for sintering (densification) result in decreases inthe strengths of SiC fibers. However, in the current invention, heattreatment of fibers in a nitrogen atmosphere at temperatures above thesintering temperature results in fibers with high tensile strength.

It is known from U.S. Provisional Appl. Ser. No. 60/055,424, thedisclosure of which is incorporated herein by reference, that heattreatment of boron-doped SiC fibers in a nitrogen gas-containingatmosphere at temperatures above the sintering temperature will resultin a BN coating on the surface of the fibers. These fibers have highstrength and high creep resistance. In the current invention, SiC fiberswith high strength and creep resistance are prepared without the BNcoating by also using heat treatment in a carbon monoxide (CO)gas-containing atmosphere, which removes boron and boron nitridecoatings from the fibers. The latter heat treatment can be carried outbefore or after the heat treatment in the nitrogen gas-containingatmosphere. The latter heat treatment is carried out at temperaturesabove the sintering temperature or for longer timed than used forsintering (densification.) The heat treatment in the CO gas-containingatmosphere may occur at temperatures which are lower, comparable, orhigher than the sintering temperature (and/or for times which areshorter, comparable, or longer than the sintering time), but typicallybetween 1600 and 2200 degrees C., more preferably between 1700 and 2000degrees C., and most preferably between 1700 and 1900 degrees C.

The changes which occurs in the SiC fiber structure during the variousheat treatments described in this invention are complex. It is notnecessary to have a detailed mechanistic understanding of these changesto practice the invention. Nevertheless, it may be helpful inunderstanding the scope of the invention to postulate the reasons forthe high creep resistance and high strength of the fibers preparedaccording to the methods of this invention.

It is believed that the creep resistance of the SiC fibers prepared bythis invention are improved largely due to grain growth when heattreatment is carried out at higher temperatures and/or longer times thanused in the sintering process. Transmission electron microscopeobservations showed that the grain sizes increased when fibers were heattreated in nitrogen at temperatures above the sintering temperature.(Larger grain size would also be expected for heat treatments carriedout at the sintering temperature but for much longer times than requiredfor densification.) According to many scientific publications, the creeprate due to bulk and grain boundary diffusional processes decreases asthe grain size increases (i.e., improved creep resistance is observedwith increased grain size). SiC ceramics are generally believed toundergo creep by diffusional processes, so it is not surprising thatimproved creep resistance is observed with increasing grain size.Another possible factor which may contribute to the improved creepresistance of the SiC fibers in this invention is that some nitrogenatoms may dissolve (as a solution) into the lattice structure of the SiCgrains during the heat treatment in the nitrogen containing atmosphere.This incorporation of nitrogen atoms within the SiC lattice is expectedto cause a decrease in the SiC diffusion rates (and, therefore, adecrease in the creep rate).

The SiC fibers in this invention retain high strength after the heattreatments despite the increased grain size. It is postulated that thisoccurs due to changes in the surface structure of the SiC fibers. In SiCfibers that are processed according to U.S. Pat. No. 5,792,416 and areonly given a standard sintering heat treatment, the strength is believedto be controlled by the size of the SiC grains. Fractography revealedthat failure during tensile testing often is initiated at the fibersurface. It is believed that larger grains intersecting the surface actas strength-limiting flaws. When fibers are heat treated at temperaturesabove the sintering temperature in an inert atmosphere such as argon,the flaw sizes that control the strength increase because the grainsincrease in size. Hence, the tensile strengths are observed to decreasein such cases. However, it is believed that the heat treatments in thegas atmospheres used in this invention change the nature of the surfaceso that the strength is no longer controlled by the larger SiC grainsthat form during heat treatment. The larger SiC grains are not presentat the immediate surface of the fiber and, hence, the flaw sizes at thesurface remain small. For example, in U.S. Provisional Appl. Ser. No.60/055,424, when the boron-doped SiC fibers are heat treated above thesintering temperature only in a nitrogen-containing gas atmosphere, thefibers have high strength. This heat treatment results in a BN coatingon the fibers. The thickness of the coating is typically less than orequal to several tenths of a micrometer. This thickness is smaller thanthe size of most of the SiC grains in the heat-treated fibers. Inaddition, the BN grains in the BN coating have small size. For thesereasons, the flaws at the surface remain small and the fiber strengthremains high. Similarly, in the present invention, the heat treatmentsin the nitrogen and CO atmospheres also produce a surface layer in whichthe flaws remain small. This surface layer in this invention, however,is not BN as in U.S. Provisional Appl. Ser. No. 60/055,424. Thisinvention indicates that any heat treatment which produces a surfacestructure which can limit the strength-limiting flaws to sizes smallerthan the typical grains sizes should produce fibers with high tensilestrengths.

Another possible factor which may contribute to the retention of hightensile strength of the fibers after the heat treatments in thisinvention is that the heat treatments may introduce compressive stresseson the surface, possibly due to the difference in thermal expansioncoefficient between the near-surface region of the fiber and the SiCinterior of the fiber. There is also a possibility that near-surfacecompressive stresses may also develop if some nitrogen atoms dissolve inthe SiC lattice during the heat treatment in the nitrogen.

This invention may be practiced such that all the heat treatments can becarried out together as part of a combined process for fabricating thefibers, i.e., it is not necessary to remove the fibers from the furnaceeach time the heat treatment involves a different gas atmosphere and/ortemperature. The atmospheres and temperatures in the furnace can simplyby changed in a sequential fashion during one continuous furnace run.Thus, it is not necessary to carry out distinct and separate furnaceheat treatments for fiber fabrication and fiber surface modification.

Alternatively, the heat treatments can be carried out on pre-existingSiC fibers. Hence, the method could be applied to improve the propertiesof pre-existing fibers such as HI-NICALON TYPE S™ produced by NipponCarbon Co. and SYLRAMIC™ produced by Dow Corning Corp. and any otherappropriate commercially produced or developmental SiC fiber.

The process is demonstrated in the examples below for SiC fibersprepared by dry spinning of polycarbosilane-based polymers, but it isevident that the process can be applied to other SiC fiber fabricationprocesses, including polymer-derived fiber fabrication processes basedon melt spinning, wet spinning, or dry spinning of other organosiliconpre-ceramic polymers. The process can be applied to SiC fibers preparedby fabrication methods based on powder processing and vapor-phaseprocessing. Furthermore, the process can be applied to other SiCceramics such as bulk samples, substrates, coatings, etc.

Two types of measurements were carried out to demonstrate thesimultaneous development of high strength and high creep resistance inthe SiC fibers produced in this invention: (i) tensile strength testsand (ii) Morscher-DiCarlo bend stress relaxation (BSR) tests. Thetensile strength measurements were carried out according to the ASTM(American Society for Testing and Measurements) procedure D3379. The BSRtests were carried out using the method developed by Morscher andDiCarlo (J. Am. Ceram. Soc., 75[1] 136-140 1992). This test involvessubjecting the fiber to heat treatment under a fixed stress and thendetermining a stress relaxation ratio, designated as the "M" value. Thisvalue is considered to be a measure of creep resistance of the fiber. Itdepends on the various testing conditions (the time/temperature scheduleusing during heat treatment, the stress on the fibers during heattreatment, etc.) and the fiber characteristics. M takes on valuesbetween 0 and 1. For a given stress and time/temperature heat treatmentschedule, fibers with better creep resistance will have M values closerto 1. As will be described below, the SiC fibers in this invention showhigher M values compared to other SiC fibers tested under the sameconditions (1400 degrees C. in argon for one hour).

EXAMPLES Example 1 (A Comparative Example)

The first example describes the preparation of a control fiber sample,i.e., a boron-doped SiC fiber which was prepared using only theprocedures described in U.S. patent application Ser. No. 08/683,475.Polycarbosilane (PCS) polymers with molecular weights of approximately10,000 and 11,000 were synthesized by pressure pyrolysis ofpolydimethylsilane (PDMS) according to the procedure of Toreki et al. inU.S. Pat. No. 5,171,722. Polyvinylsilazane (PSZ) was prepared accordingto the procedures of Toreki et al. in U.S. Pat. No. 5,171,722 and in"Synthesis and Applications of a Vinylsilazane Preceramic Polymer,"Ceram. Eng. Sci. Proc., 11 [9-10] 1371-1386 (1990). Polyvinylsiloxane(PSO) were prepared according to the same procedures as used for PSZpreparation, except that a siloxane monomer was used instead of asilazane monomer in the polymerization reaction. Solid boron hydride(SBH), dicumyl peroxide (DCP), and toluene were obtained commerciallyand used without any modification. Two separate spinning solutions wereprepared with the same composition using the aforementioned PCS polymerswith molecular weights of approximately 10,000 and 11,000. Each solutionwas prepared by mixing 9.0 mL toluene, 3.8 g PCS, 0.294 g of PSO, 0.042g PSZ, 0.063 g SBH, and 4 mg DCP. The solutions were filtered and thenconcentrated by rotary evaporation to increase the viscosity until thesolutions had solvent (toluene) concentration in the range ofapproximately 30-35%. The concentrated polymer solutions were used toform "green" fibers by the conventional dry spinning method. The "green"(as-collected) fibers were heat treated according to the procedures inU.S. Pat. No. 5,792,416 in order to pyrolytically decompose theorganometallic precursor polymers (thereby forming a substantially SiCceramic) and to subsequently densify the SiC fiber via sintering.Sintering heat treatments were carried out using a maximum temperatureof ˜1840-1845° C. for 12 minutes in a flowing argon atmosphere.

The fibers produced by the above method had an average tensile strengthof ˜2.8 GPa. The average fiber diameter was ˜10 μm. The fibers contained˜1 wt % boron, as determined by neutron activation analysis. BSR testswere carried out using heat treatment at 1400° C. for 1 hour in an argonatmosphere. The average M value under these conditions was ˜0.5.

As noted earlier, it is generally undesirable to sinter fibers at highertemperatures higher and/or longer times than required to achievedensification. This was demonstrated with the control SiC fibers bycarrying out heat treatment as follows. The fibers were initially heattreated in the same fashion as the sample described above. However,after heating at 1840° C. for 12 minutes in an argon atmosphere, theheat treatment was continued at this temperature for an additional 5minutes and then the temperature was raised to 1940° C. whilemaintaining the argon atmosphere. The fibers were held at 1940° C. for 1hr in the argon atmosphere and then the furnace was cooled to roomtemperature. The fibers had an average diameter of ˜10 μm. The averagetensile strength was ˜2.1 GPa, i.e., a 25% decrease in tensile strengthfrom the value obtained for the control fiber samples which were onlyheat treated at 1840-1845° C. for 12 minutes. The decrease in strengthis attributed to grain growth. Transmission electron microscopy (TEM)observations showed that grains grow to a larger size when fibers wereheat treated to temperatures above the sintering temperature.

Example 2 (A Comparative Example)

BN-coated samples were prepared using the same batches of "green" fibersthat had been prepared for EXAMPLE 1. The green fibers were initiallyheat treated in the same way as the fibers prepared in EXAMPLE 1.Sintering was carried out at ˜1840-1845° C. for 12 minutes in an argonatmosphere. However, after the 12 minute hold period, the flow of argongas into the furnace was discontinued and the furnace was filled withnitrogen gas. (A flowing atmosphere is maintained during all the heattreatment operations such that the gas entering into the furnace forcesout the gas which is already within the furnace. Hence, the argonatmosphere was quickly replaced by a nitrogen gas atmosphere.) Thetemperature was held at ˜1840-1845° C. for an additional 5 minutes afterthe flow of argon gas into the furnace was discontinued and the flow ofnitrogen into the furnace was started. The temperature was thenincreased to 1890° C. while maintaining the nitrogen atmosphere. Thesamples were held at 1890° C. for 10 minutes in the nitrogen atmosphereand then the furnace was cooled to room temperature.

The SiC fibers produced with BN coatings by the above method hadessentially the same average tensile strength (within experimentalerror) as the control (uncoated) SiC fibers in EXAMPLE 1 which weresintered at 1840-1845° C. in argon for 12 minutes. The fibers heattreated in nitrogen at 1890° C. for 10 minutes (in this example) had anaverage tensile strength of ˜2.9 GPa and an average diameter of ˜10 μm.

The development of a BN coating on the SiC fibers heat treated in anitrogen atmosphere was proven using the surface characterizationtechnique of Auger Electron Spectroscopy (AES). For comparison, FIG. 1shows an AES spectrum for the control (uncoated) SiC fiber sample fromEXAMPLE 1 which was heat treated at 1845° C. for 12 minutes in argon.Carbon (C) is the major element observed in the spectrum. Some silicon(Si) and oxygen (O) peaks are also observed. The control fiber samplehas a thin, highly carbon-rich layer. It is known from publishedscientific studies that when silicon carbide is heat treated at hightemperatures in inert or reducing atmospheres, some silicon tends topreferentially evaporate from the near surface region. (In addition,there is a tendency for some extra carbon (so-called "adventitious"carbon) to contaminate the surface of samples during AES studies. Theoxygen peak in the spectra in FIG. 1 is also believed to be due surfacecontamination, probably coming from the mounting material (an adhesive)used to hold the fibers in place when the AES chamber was evacuated.This is believed because it is known from bulk chemical analyses thatthe overall oxygen content of the fibers is less than ˜0.1 wt %.) FIG. 2shows the AES spectrum for the same control (uncoated) SiC fiber afterit had been subjected to sputtering with argon in order to remove thethin layer of carbon from the SiC fiber surface. The spectrum nowappears more representative of the bulk SiC fiber, i.e., the Si:C ratiois close to that of stoichiometric SiC.

FIG. 3 shows the AES spectrum for the BN-coated SiC fiber prepared byheat treatment at 1890° C. for 10 minutes in a nitrogen atmosphere. Themajor peaks observed are for B and N. A small peak is observed for C,but this might be due to adventitious carbon contamination. FIG. 4 showsthe AES spectrum for the same BN-coated fiber (prepared by heattreatment at 1890° C. for 10 minutes in a nitrogen atmosphere) after ithad been subjected to sputtering with argon in order to remove a thinlayer of the SiC fiber surface. The only significant peaks in thespectrum are for B and N. (A small peak for argon is observed whichcomes from using argon for sputtering.)

Additional BN-coated SiC fiber samples were prepared using the sameprocedure as described above except that the heat treatment temperaturewas changed. Sintering was still carried out at ˜1840-1845° C. for 12minutes in an argon atmosphere. After the 12 minute hold period, theflow of argon gas into the furnace was discontinued and the furnace wasfilled with nitrogen gas. The temperature was held at ˜1840-1845° C. foran additional 5 minutes after the flow of argon gas into the furnace wasdiscontinued and the flow of nitrogen into the furnace was started. Thetemperature was then increased to 1940° C. while maintaining thenitrogen atmosphere. The samples were held at 1940° C. for 1 hr in thenitrogen atmosphere and then the furnace was cooled to room temperature.

FIG. 5 shows the AES spectrum for the unsputtered surface for theBN-coated SiC fiber prepared by heat treatment at 1940° C. for 1 hour ina nitrogen atmosphere. The spectrum is essentially identical to thespectrum in FIG. 3 for the unsputtered surface for the BN-coated SiCfiber prepared by heat treatment at 1890° C. for 10 minutes in anitrogen atmosphere. The major peaks observed are for B and N.

The fibers heat treated in nitrogen at 1940° C. for 1 hour had anaverage tensile strength of ˜2.9 GPa and an average diameter of ˜10 μm,i.e., essentially the same values as obtained for the control samplefibers in EXAMPLE 1 which were sintered at 1840-1845° C. in argon for 12minutes. However, the SiC fibers produced with BN coatings by the abovemethod have greatly improved creep resistance, as indicated by BSRtests, compared to the control (uncoated) SiC fibers in EXAMPLE 1 whichwere sintered at 1840-1845° C. in argon for 12 minutes. The average Mvalue obtained from a BSR test at 1400° C. for 1 hour in argon was ˜0.9for the sample heat treated at 1940° C. for 1 hour in a nitrogenatmosphere. This M value is similar or higher than reported forrelatively low-strength, powder-derived fibers (such as Carborundum SiCfibers produced by the method of Frechette et al.).

Example 3 (A Comparative Example)

EXAMPLE 2 showed that BN-coated SiC fibers with high tensile strengthare developed when the fibers are heat treated in nitrogen at atemperature above the sintering temperature. In contrast, this exampleshows that using a nitrogen atmosphere for the sintering (densification)stage, which has been practiced sometimes in the prior art, will notproduce fibers with high tensile strength.

Fibers were prepared using the same batches of "green" fibers that hadbeen prepared for EXAMPLE 1. The green fibers were heat treated in thesame way as the fibers prepared in EXAMPLE 1 up through the initialpyrolysis, i.e., up to a heat treatment temperature of 1150° C. (whichis sufficient to pyrolytically decompose the organosilicon polymer to anSiC-based ceramic). Above 1150° C., the fiber in this example was heattreated in a nitrogen atmosphere, instead of the argon atmosphere usedin EXAMPLE 1. The fibers were first heat treated at ˜1840-1845° C. for12 minutes in the nitrogen atmosphere. After the 12 minute hold period,the temperature was increased to 1870° C. while maintaining the nitrogenatmosphere. The samples were held at 1870° C. for 10 minutes in thenitrogen atmosphere and then the furnace was cooled to room temperature.The fibers heat treated in this manner had an average tensile strengthof only 1.4 GPa. The average diameter of the fibers was ˜11 μm.

Example 4

Fibers were prepared using the same batches of "green" fibers that hadbeen prepared for EXAMPLE 1. The green fibers were initially heattreated in the same way as the fibers prepared in EXAMPLE 1. Sinteringwas carried out at ˜1840-1845° C. for 12 minutes in an argon atmosphere.However, after the 12 minute hold period, the flow of argon gas into thefurnace was discontinued and the furnace was filled with nitrogen gas.The temperature was held at ˜1840-1845° C. for an additional 5 minutesafter the flow of argon gas into the furnace was discontinued and theflow of nitrogen into the furnace was started. The temperature was thenincreased to 1890° C. while maintaining the nitrogen atmosphere. Thesamples were held at 1890° C. for 10 minutes in the nitrogen atmosphere.The temperature was then decreased to 1700° C. while maintaining thenitrogen atmosphere during cooling. Upon reaching 1700° C., the flow ofnitrogen gas was discontinued and the furnace was filled with a gashaving an approximate composition of 15% carbon monoxide (CO)/85% argon.The samples were held at 1700° C. for 30 minutes in the 15% carbonmonoxide (CO)/85% argon atmosphere. The atmosphere was then switched toargon only and the furnace was cooled under the argon atmosphere.

The SiC fibers produced under the above conditions had an averagetensile of 3.0 GPa and an average diameter of ˜10 μm. The average Mvalue obtained from a BSR test at 1400° C. (for 1 hour in argon) was˜0.81 which is similar to powder-derived SiC fibers (such as CarborundumSiC fibers produced by the method of Frechette et al.). Hence, SiCfibers with high tensile strength and significantly improved creepresistance (as indicated by the M value from the 1400° C. BSR test) havebeen developed relative to the fibers in EXAMPLE 1 (which were producedusing only the steps indicated in U.S. patent application Ser. No.08/683,475). In addition, the SiC fibers produced by this method showimproved creep resistance compared to other high-strength SiC fibers.For example, DiCarlo (Composites Science and Technology, 51 213-222,1994) and Takeda et al. (Ceram. Eng. Sci. Proc., 17[4] 35-42, 1996) havereported M values in the range of approximately 0.5-0.6 for SYLRAMIC™fibers and HI-NICALON TYPE S™ fibers in BSR tests carried out at 1400°C. for 1 hour in argon. As noted previously. Takeda et al. showed thatthe M values for the HI-NICALON TYPE S™ fibers can be increased (i.e.,to similar values as obtained for the SiC fibers in this example) byannealing at temperatures above the sintering temperature. However, thestrength of such annealed fibers prepared by Takeda et al. decreasedvery significantly upon annealing (i.e., to values ≦1.5 GPa).

FIG. 6 shows the AES spectrum for the unsputtered surface for the SiCfiber prepared according to the above conditions. The only significantelement detected at the surface of the fiber was carbon (C). This shouldbe compared to the spectrum in FIG. 3 (EXAMPLE 2) which is for theunsputtered surface for the SiC fiber prepared by the same heattreatment conditions as used for the fibers in this example except thatthe heat treatment step in the CO gas-containing atmosphere was notcarried out for EXAMPLE 2. The spectrum in FIG. 3 shows that the SiCfiber in EXAMPLE 2 developed a BN coating on the surface. It is evidentfrom FIG. 6 that there is no BN coating on the SiC fiber heat treated inthe CO-containing gas atmosphere. In U.S. Provisional Appl. Ser. No.60/055,424, it was shown that heat treatment in a CO-containingatmosphere can remove boron from SiC fibers. In the present invention,it is now shown that heat treatment in the CO-containing gas atmospherecan be used to remove boron nitride (BN) coatings.

The fibers produced in this example were also subjected to argonsputtering in order remove a thin surface layer. AES on the new surfacelayer showed that some B and N remain in the fibers.

Example 5

Fibers were prepared using the same batches of "green" fibers that hadbeen prepared for EXAMPLE 1. The green fibers were initially heattreated in the same way as the fibers prepared in EXAMPLE 1, except thatsintering was carried out at ˜1840-1845° C. for 15 minutes (instead of12 minutes) in an argon atmosphere. After this 15 minute hold period,the flow argon gas was discontinued and the furnace was filled with agas having an approximate composition of 15% carbon monoxide (CO)/85%argon. After a 50 minute hold period at the same temperature, the flowof the 15% carbon monoxide (CO)/85% argon gas was discontinued and thefurnace was filled with nitrogen gas. After another 10 minute holdperiod at the same temperature, the temperature was increased to 1940°C. while maintaining the nitrogen atmosphere. The samples were held at1940° C. for 30 minutes in the nitrogen atmosphere. The furnace was thencooled to room temperature while maintaining the nitrogen atmosphere.

The SiC fibers produced under the above conditions had an averagetensile of ˜2.8 GPa and an average diameter of ˜10 μm. The average Mvalue obtained from a BSR test at 1400° C. (for 1 hour in argon) was˜0.9. Hence, SiC fibers with high tensile strength and significantlyimproved creep resistance (as indicated by the M value from the 1400° C.BSR test) have been developed relative to the fibers in EXAMPLE 1 (whichwere produced using only the steps indicated in U.S. patent applicationSer. No. 08/683,475). In addition, the fibers produced by this methodshow improved creep resistance compared to other high-strength SiCfibers, such as SYLRAMIC™ fibers and HI-NICALON TYPE S™ fibers.

FIG. 7 shows an AES spectrum for the unsputtered surface for the SiCfiber prepared according to the above conditions. The primary elementdetected at the surface of the fiber was carbon (C). A small amount ofnitrogen (N) is also present. It is evident that the fibers do not havea BN coating.

The illustrations set forth above are by way of example only, and thetrue scope and definition of the invention is to be as set forth in thefollowing claims.

I claim:
 1. A method of forming a high strength, high creep resistance,silicon carbide fiber having no boron nitride coating, comprising thesteps of:(A) providing a boron-doped, silicon carbide fiber which hasbeen sintered at a sintering temperature to densify the fiber; (B)exposing the fiber to a nitrogen-containing atmosphere at a temperatureat least equal to the sintering temperature; (C) exposing the fiber toan atmosphere containing carbon monoxide at a temperature sufficient toremove boron and boron nitride from the fiber.
 2. The method of claim 1,where said fiber is first exposed to the nitrogen-containing atmosphereand secondly exposed to the carbon monoxide-containing atmosphere. 3.The method of claim 1, where said fiber is first exposed to the carbonmonoxide-containing atmosphere and secondly exposed to thenitrogen-containing atmosphere.
 4. The method of claim 2, where thetemperature of the fiber in the nitrogen-containing atmosphere is atleast approximately 45 degrees C. above the sintering temperature. 5.The method of claim 4, where the temperature of the fiber in the carbonmonoxide-containing atmosphere is lowered to at least the sinteringtemperature.
 6. The method of claim 3, where the temperature of thefiber in the carbon monoxide-containing atmosphere is approximatelyequal to the sintering temperature.
 7. The method of claim 6, where thetemperature of the fiber in the nitrogen-containing atmosphere is atleast approximately 100 degrees C. above the sintering temperature. 8.The method of claim 1, where said fiber has a density of at leastapproximately 3.0 g/cm³.
 9. The method of claim 1, where saidnitrogen-containing atmosphere is 100 percent nitrogen.
 10. The methodof claim 1, where said carbon monoxide-containing atmosphere has atleast about 15 percent carbon monoxide.
 11. The method of claim 1, wherethe fiber is exposed to said atmosphere containing nitrogen at atemperature between approximately 1750 and 2250 degrees C. and to saidatmosphere containing carbon monoxide at a temperature betweenapproximately 1600 and 2200 degrees C.
 12. The method of claim 1, wherethe fiber is exposed to said atmosphere containing carbon monoxide at atemperature between approximately 1700 and 1900 degrees C.
 13. A methodof forming a high strength, high creep resistance, silicon carbide fiberhaving no boron nitride coating, comprising the steps of:(A) providing aboron-doped, silicon carbide fiber; (B) exposing the fiber to anatmosphere containing nitrogen at a temperature between approximately1750 and 2250 degrees C.; and (C) exposing the fiber to an atmospherecontaining carbon monoxide at a temperature between approximately 1600and 2200 degrees C.
 14. The method of claim 13, where saidnitrogen-containing atmosphere is 100 percent nitrogen.
 15. The methodof claim 13, where said carbon monoxide-containing atmosphere has atleast about 15 percent carbon monoxide.